Method and device to embed watermark in uncompressed video data

ABSTRACT

A system and method to watermark an uncompressed content received at a client device in a compressed form and encrypted by at least one content key, said content key as well as watermark instruction forming Conditional Access Module (CAS) data, said CAS data being encrypted by a transmission key and comprising at least one signature to authenticate the CAS data, said client device comprising a CAS configured to process the CAS data, a descrambler having an input to receive the encrypted compressed content and an output to produce an compressed content, a decoder to convert the compressed content into uncompressed content, a watermark inserter connected to the output of the decoder, a secure activation module connected with the watermark inserter, a secure element connected with the watermark inserter and the secure activation module.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2014/057342 filed Apr. 11, 2014, which claimspriority from European Patent Application No. 13165591.2 filed Apr. 26,2013.

BACKGROUND ART

Using watermarking solution on the un-compressed domain is elegant asthe watermarking is done standalone in the client device, and as done inthe un-compressed domain, it generally offers the best watermarkinsertion's capability and better invisibility.

But working after the decoder (for un-compressed watermarking) offers nointeraction with the security elements which are linked to thedescrambling of the data, thus occurring before the decoder.

The document U.S. 2010/128871 describes a solution in which a secondarystream is generated comprising the data allowing to reconstruct the mainstream and at the same time to marl the reconstructed stream. As aconsequence, the same stream contains the scrambling data and thewatermarking data. At the reception, this stream is processed as one setof data to be replaced in the modified stream.

The document EP 2 391 125 describes a solution to allow an individualmarking (at the reception device) based on a stream common to alldevices. The control object contains the original value, an alternatevalue and a location. The security unit determines a mathematicaloperation to be applied on the alternate value to retrieve the originalvalue. The mathematical operation is changed according to an internalparameter of the reception device so that the mathematical operationwill be unique per device, allowing to track this device if the streamof descrambling data is analyzed.

BRIEF DESCRIPTION OF THE INVENTION

In the proposed solutions, the same content object comprises the key todescramble the main content and the marking data. This is only suitablewhen the same element is in charge of both operations and needs aparticular descrambler module able to execute both operations.

The aim of the present invention is to enforce the watermarking on acontent received by a client device, in particular when the element incharge of the descrambling is independent of the element in charge ofthe watermarking.

The present invention concerns a method to watermark an uncompressedcontent received at a client device in a compressed form and encryptedby at least one content key (CW), said content key (CW) as well aswatermark instruction forming CAS data, said CAS Data being encrypted bya transmission key and comprising at least one signature to authenticatethe CAS Data, said client device comprising:

-   -   A Conditional Access Module (CAS) in charge of the CAS Data,    -   a Descrambler (DSC) having an input to receive the encrypted        compressed content and an output to produce an compressed        content,    -   a decoder to convert the compressed content into uncompressed        content,    -   a Watermark inserter (201) connected to the output of the        Decoder,    -   a Secure Activation module (202) connected with the Watermark        inserter (201),    -   a Secure Element (SE 200) connected with the Watermark inserter        (201) and the Secure Activation module (202),        said method comprising the following steps:    -   receiving the CAS Data,    -   decrypting by the Conditional Access Module (CAS) the CAS Data        with the transmission key,    -   verifying the signature of the CAS Data, and if the signature is        valid,    -   transferring the content key (CW) to the descrambler (103) and        the watermark instruction to the Secure Activation module (202),    -   transferring the watermark instruction and the unique identifier        to the Watermark inserter (201),    -   enabling or disabling watermarking by the Watermark inserter        (201), the uncompressed content received from the Decoder by        watermarking data initialized by the unique identifier.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be better understood thanks to the attachedfigures in which:

FIG. 1 illustrates a Watermarking on Un-Compressed Content just afterthe Video Decoder,

FIG. 2 illustrates a Watermarking on Un-compressed Content into theHDMI,

FIG. 3 illustrates an exchange of messages between the SE and the SA,

FIG. 4 illustrates an alternate embodiment of the exchange of messagesbetween the SE and the SA.

DETAILED DESCRIPTION

Conditional access data comprises a content (video or audio data orcombination thereof) encrypted by one or a plurality of content keys,and comprises CAS data, i.e. data allowing to decrypt the encryptedcontent. The CAS data also comprises conditional access conditionsassociated with the content describing the condition to be met by thedecoder to decrypt the content. The conditional access data arebroadcasted, unicasted or sent on request of the recipient. The CAS Datacan also comprise a watermark instruction to be passed to theWatermarking Inserter.

Once received in the decoder, the CAS data are extracted from theconditional access data and forwarded to a Conditional Access module(CAS in FIGS. 1 and 2), in charge of the enforcement of the securitymeasures defined in the conditional access conditions. This module canhave several forms, i.e. a physical module or a software module. A wellknown solution is in the form of a smart card with interface 7816, butthe present invention applies to other form factor or interface such asUSB, LVDH, PCMCIA. The module CAS can be also integrated into the secureelement SE.

This module comprises the key necessary to decrypt the CAS data and toextract from the CAS data the key (CW) to decrypt the content. Inaddition to the key, this module extracts the conditions (usage rules)and transfers the key and the conditions to a secure element SE. It isto be noted that the communication between the CAS and the SE isencrypted and protected by a key initialized on both elements.

The messages exchanged between the CAS and the SE could be signed, i.e.a digest of the payload (e.g. watermarking instruction, the randomnumber and the identifier) is calculated by the CAS (using for examplehash function) and encrypted by a signature key to produce thesignature. This signature is added into the message and the message ispreferably encrypted by a transmission key. On the reception side, themessage is first decrypted and the digest is calculated on the payload.With the key corresponding with the signature key, the receivedsignature is decrypted and the result is compared with the calculateddigest. If both values are the same, the message is authentic. Thesignature key can be a symmetric key or an asymmetric key(public/private keys).

In a conventional environment, the key obtained by the CAS istransferred to the descrambler DSC in charge of decrypting the content.Once the content is decrypted, the content is processed by a decoder toproduce an uncompressed content. A watermarking module WMK receives theuncompressed content and applies the watermark, this watermark beingparameterized by a unique identifier of the decoder.

The proposed solution is to have a secure link between the secureelement (SE) 200 (refer to FIG. 1) provided by the CAS environment andthe un-compressed domain watermarking element 201.

According to FIG. 1, the SE 200 relays the CAS data, which introducesthe explicit dependence between the successful descrambling operationand the activity of the block located after the decoder, namely theWatermarking Inserter.

The SE 200 also supplies other sensitive information (like a unique IDof a device, the CAS module ID, an ID contained in the CAS data orkeys). This information, if transmitted in clear to the WatermarkingInserter could be intercepted and modified. This is why the connectionbetween these two blocks should be secured.

The Watermarking Inserter 201 is here to insert an information (thewatermark payload) into the audio and/or video content. This payload isdependant of the unique identifier received from the CAS. It can then bebypassed by a hacker (the audio and/or video content is not sent to thewatermarking bloc), or the payload (the watermark data) can be modified.This watermarking inserter can be positioned just after the decoder, oralong the line of transmission between the decoder and the finaldisplay, like the HDMI transmitter, or the TV screen.

The proposed solution ensures:

-   -   A way to ensure the Watermarking Inserter 201 is not bypassed        and therefore activated if decided by the CAS. If the        Watermarking Inserter 201 doesn't seem to be activated, then the        SE 200 stops descrambling the content.    -   A way to ensure communication between SE 200 and Watermarking        Inserter 201, so that the Watermarking Inserter 201 accesses        with certitude to the information it requires at a moment T such        as the necessary payload to insert into the content. The SE 200        manages all or part of the computations on the information to be        inserted. Generally speaking this information is: the Unique        Identifier (UID) representing the decoder or the CAS module, a        time stamp (T), and any other information of interest like a        specific information (SID) on the content itself. From this data        a payload is computed before the insertion into the content.        This operation aims at protecting the payload against attacks or        transmissions errors (computations through an ECC or an        anti-collusion code like Tardos code).

Then all or part of this payload has to be used by the watermarkingsystem to be inserted into the content.

The proposed solution is to add a Secure Activation block 202 whichactivates/deactivates the Watermarking Inserter 201. For securityreason, it is recommended to implement the Secure Activation block 202and the Watermarking Inserter 201 just after the Video decoder and in aTrusted Execution Environment (TEE). The access between the SecureActivation block 202 (SA 202) and the Watermarking Inserter 201 is notaccessible from the host CPU of the client device.

The Secure Activation block 202 is controlled by some registersaccessible, for example, either via a private bus as illustrated in FIG.1 or via a generic bus (AMBA APB) as illustrated in FIG. 2. The SecureActivation block 202 is able to at least enable/disable two features:watermarking on video content, watermarking on audio content, orcombination thereof. In FIG. 1 and FIG. 2, the Secure Activation block202 and the Watermarking Inserter 201 are connected on the videocontent.

The main concept relies on the fact that the Secure Activation block 202has to receive a message called Activation Message (AM) from the SE 200to activate/deactivate the different features. The AM is also used totransmit the payload necessary to mark the content. Another featurecarried out by the Secure Activation block 202 is a secure link with theSE 200 to exchange sensitive information. For example, the SE 200 couldtransmit secure settings to the Secure Activation block 202. Then theSecure Activation block 202 will transmit them to the Watermarking bloc201.

The principal attack is that a person records this message and thereforeis able to activate or deactivate these features as he wishes. Thisattack is called “replay attack”.

The second attack is that an attacker tries to modify the content of themessage. Therefore when the SE 200 generates an AM, it shall beencrypted and signed for the Secure Activation block 202. In addition,the AM shall embed a random value coming from a True Random NumberGenerator (TRNG). An example of the AM is illustrated in FIG. 3. Itshall at least contain a random value, the activation flag, the payload(containing the UID), the payload size and the signature.

FIG. 2 describes an example where a device is using the HDMI and theWatermarking bloc 201 on the video must be activated. The ConditionalAccess System (CAS) will receive CAS data. According to this example,the CAS Data are in the form of Entitlement Control Messages ECM. TheseECMS contain an encrypted CW and its usage rules. The encrypted CW andits usage rules are transmitted to the SE 200. The usage rules indicateto the SE 200 that the current video content must be watermarked.

This SE 202 and the SA 202 share a common key stored in each device atan initialization stage. This key will be used to encrypt the messagesexchanged so that the content cannot be accessed by a third party. Sincethe communication between these two elements can be intercepted by athird party, it is important to implement a mutual authenticationprotocol. Each party should be sure to talk to the other party and notwith a simulator.

The message transmitted to the Secure Activation module (202) contains awatermark instruction. In case of an activation, the identifier to beembedded into the content is included in this message.

Then the following steps could be executed in accordance with the FIG.3.

-   -   1. The CAS receives an ECM containing an encrypted Control Word        (CW) and its usage rules or access conditions.    -   2. The SE 200 decrypts the CW and its usage rule. The usage rule        says for instance that the content shall be decrypted using the        DVB-CSA descrambler and the watermarking shall be activated on        the video. At the same time, the SE 200 generates the Activation        Message (AM) and add a variable value. This variable value        should change for each message generated by the Secure        Activation block 202. It could be a random value, a counter        incremented at each message or a time stamp. This value is        stored into the SE 200. The AM is encrypted and signed. Then the        AM is sent to the Secure Activation block 202.    -   3. The Secure Activation block 202 decrypts and authenticates        the AM. If the authentication succeeds, the Secure Activation        block 202 checks if the variable value to determine if the        received value has changed compared to a previous value. In case        of a counter, the received variable value should be higher than        the last received variable value. The same applies for a time        stamp, the new received time stamp should be in advance versus        the time stamp received in a previous message. For a random        value, the Secure Activation block 202 should store all        previously received random values (or at least a predefined        number e.g. the last 20) in order to compare the one received        with the one contained in the message.

In case that the variable value does not comply with one of the abovementioned rules, the message is rejected. Otherwise, the SecureActivation block 202 activates the features depending on the content ofthe AM. After the authentication, the Secure Activation block generatesan Acknowledge Message (ACKM) which is encrypted and signed. This ACKMis sent back to the SE 200. It shall at least contain the same randomvalue as the AM, and could also contain the activation flag, the payload(UID), the payload size, the ACK flag and the signature. The randomnumber is then stored by the Secure Activation block 202 in order toavoid that the same message is presented later.

-   -   4. The SE 200 decrypts and authenticates the ACKM. The ACKM        shall contain the same random value than the one in the AM. If        the ACKM is not correctly authenticated, not received on time        (using a watchdog), or the random value is not the same than        into the previous AM, the transfer of the watermark instruction        is considered as failed and the SE 202 stops providing the CW to        the descrambler.

It is common that the Secure Activation block 202 does not comprisepermanent memory. It is therefore not possible to store the lastvariable value for an accurate anti-replay verification. This is why theFIG. 4 illustrates another method. It is to be noted that this methodcan be applied even if the SE 202 contains permanent memory.

-   -   1. The SE 200 sends a first message (AM) to the Secure        Activation block 202 informing that an instruction message will        be transmitted. At that stage, the first message can be        encrypted or in clear. The message can contain a signature        allowing the SA 202 to authenticate the first message. In the        example of the FIG. 4, the message is an activation message AM.    -   2. In case that the message is encrypted and signed, the SA 202        decrypt and verifies the signature. If the signature is correct,        the SA 202 then generates a random number X and sends a second        message ACKM containing the random number X to the SE 200. The        random number X is stored by the SA 202. At that stage, the        second message can be encrypted or in clear. This second message        is preferably encrypted and contains a signature to ensure that        only a genuine SA 202 can generate it.    -   3. While the SE 202 receives and decrypts (if encrypted) the        message ACKM, the SE 202 extracts the random number. With this        random number and the watermark instruction, the SE 202 prepares        a message, encrypts and signs it. In case that the instruction        concerns the disabling of the watermark on the content, this        message does not need to contain an identifier. This message is        sent to the SA 202.    -   4. The SA 202 then decrypts and verifies the signature. If the        signature is correct, it verifies also that the random number X        is the same as the one stored previously. If it differs, the        message is rejected. After having passed these controls, the        watermark instruction contained in the message can be executed.    -   5. The SA 202 then prepares a final acknowledgment message FACKM        to confirm the execution of the watermark instruction. This        message will contain the random number X and is encrypted as        well.    -   6. The SE 200 receives the FACKM and once decrypted, extracts        the status of the SA 202. If the SA 202 confirms the successful        completion of the watermark instruction, the SE 2002 can then        transfer the content key to the Descrambler.

The invention claimed is:
 1. A method for watermarking uncompressedcontent comprising: receiving encrypted compressed content at a clientdevice, the encrypted compressed content being encrypted under a contentkey, the client device comprising: a conditional access module (CAS)configured to process CAS data, the CAS data being encrypted by atransmission key and comprising a content key, a watermark instruction,and at least one signature to authenticate the CAS data; a descramblerbeing configured to input the encrypted compressed content and outputdecrypted compressed content; a decoder to convert the decryptedcompressed content into uncompressed content; a watermark inserterconnected to the output of the decoder; a secure activation moduleconnected to the watermark inserter; and a secure element connected tothe CAS, the descrambler and the secure activation module; receiving theCAS data at the client device; decrypting, by the CAS, the CAS data withthe transmission key; verifying, by the CAS, the signature of the CASdata and, in response to the signature being valid, transferring thewatermark instruction to the secure activation module via the secureelement; transferring the watermark instruction to the watermarkinserter; transferring the content key to the descrambler in response tothe transfer of the watermark instruction to the watermark inserterbeing successful; and switching between enabling and disablingwatermarking by the watermark inserter of the uncompressed contentreceived from the decoder.
 2. The method of claim 1, wherein thetransfer of the watermark instruction from the secure element to thesecure activation module comprises the steps of: requesting by thesecure element a random number from the secure activation module;generating the random number by the secure activation module, storingthe random number, and sending the random number to the secure element;preparing by the secure element a message containing at least thewatermark instruction, the random number, and wherein the signature ison both the watermark instruction and the random number; sending themessage to the secure activation module; verifying by the secureactivation module that the signature is authentic and that the randomnumber is the same as the random number sent previously and, in responseto the verification, accepting the watermark instruction; and sending afinal acknowledgment message to the secure element in order to informthe secure element of the successful completion of the transfer.
 3. Themethod of claim 1, wherein the secure activation module and thewatermark inserter are located in a trusted environment, and wherein theconnection between the secure activation module and the watermarkinserter is not accessible outside of the trusted environment.
 4. Themethod of claim 1, wherein the CAS data comprises a unique identifier,said identifier being transferred to the secure activation module and tothe watermark inserter together with the watermark instruction to enablethe watermarking, said identifier defining a payload to watermark thecontent.
 5. A client device comprising: a conditional access module(CAS) configured to receive and decrypt CAS data to obtain a watermarkinstruction and a content key, the CAS data being encrypted by atransmission key and comprising the content key, the watermarkinstruction, and at least one signature to authenticate the CAS data; adescrambler having an input to receive an encrypted compressed contentand an output to produce a decrypted compressed content based on thecontent key; a decoder configured to convert the decrypted compressedcontent into uncompressed content; a watermark inserter connected to theoutput of the decoder, the watermark inserter being configured to switchbetween enabling and disabling the insertion of a watermark into theuncompressed content based on the watermark instruction; a secureactivation module connected to the watermark inserter; a secure elementconnected to the CAS, the descrambler and the secure activation module;wherein the CAS is configured to verify the signature of the CAS dataand, in response to the verification of the signature, to transfer thewatermark instruction to the secure activation module via the secureelement; wherein said secure element is configured to verify that thewatermark instructions were successfully transferred to the watermarkinserter and, in response to the verification of the successful transferof the watermark instruction to the watermark inserter, to transfer thecontent key to the descrambler.
 6. The client device of the claim 5,wherein the secure activation module comprises a random number generatorconfigured to produce a random number, said secure activation modulegenerating, storing and sending said random number to the secureelement, the secure element being configured to prepare a messagecontaining at least the watermark instruction, the random number, andwherein the signature is on both the watermark instruction and therandom number, said secure activation module being configured to verifythat the signature is authentic and that the random number is the sameas the stored random number and, in response to the verification thatthe signature is authentic and that the random number is the same as thestored random number, accept the watermark instruction and send a finalacknowledgment message to the secure element in order to inform thesecure element of the successful completion of the transfer.